Printer

ABSTRACT

There is provided a printer including first to fourth head chips, and first and second rollers. The first to fourth head chips are arranged at intervals in a second direction orthogonal to a first direction. A nozzle, of nozzles arranged in a second head chip N and used for printing, which is positioned closest to the one side in the second direction is referred to as a nozzle A. A nozzle, of nozzles arranged in a fourth head chip N and used for printing, which is positioned closest to the one side in the second direction is referred to as a nozzle B. The nozzle A is arranged closer to the one side in the second direction than the nozzle B and the first roller is arranged closer to the other side in the second direction than the second roller.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent ApplicationNo. 2016-070089 filed on Mar. 31, 2016, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND Field of the Invention

The present invention relates to a printer.

Description of the Related Art

As a printer, there is conventionally known a line-type jetting headincluding head chips arranged in a width direction of a recordingmedium. In such a jetting head, two head chips are adjacent to eachother in the width direction of the recording medium and deviate fromeach other in a conveyance direction of the recording medium. Further,ends of the two head chips are arrayed in the conveyance direction ofthe recording medium.

In an area, of the jetting head, in which the two adjacent head chipsface each other in the conveyance direction, nozzles of the two headchips are used selectively in a predefined boundary position. In thatcase, if the two head chips differ in jetting characteristics, landingdeviations of ink jetted from the two head chips occur, resulting instreak-like density unevenness.

As a means of solving the above problem, there is conventionally known aprinter in which an area where two head chips face each other in aconveyance direction is divided into a section for jetting ink from onlyone of the two head chips, a section for jetting ink from only the otherof the two head chips, and a section for jetting ink from both of thetwo head chips. In the following, for easy explanation, each nozzlewhich is arranged at a boundary of the section where both of the twohead chips are used in the area where the two head chips face each otherin the conveyance direction is referred to as a “boundary nozzle”. In anarea for which printing is performed with the boundary nozzles, dropletsof ink jetted from nozzles of the respective two head chips land in adispersed or scattered state. This prevents density unevenness whichwould be otherwise caused by the difference in jetting characteristicsof nozzles between the respective two head chips.

Another publicly known printer has a jetting head in which four headchips are arranged zigzag, wherein each roller pressing a recordingmedium during printing is provided between the head chips. Namely, eachroller is arranged at a position next to one of two adjacent head chipsin a width direction of the recording medium and facing the other of thetwo adjacent head chips in a conveyance direction of the recordingmedium.

SUMMARY

Although the former printer has the section where ink is jetted fromnozzles of both of the two head chips, density unevenness of an imagepart formed by using nozzles of this section is still conspicuous, ascompared to density unevenness of an image part formed by nozzles of asingle head chip.

Thus, in a printer in which the above-described jetting heads arrangedin the conveyance direction, if positions of boundary nozzles in anozzle arrangement direction are the same between the jetting heads,printing quality might deteriorate.

Further, in order to prevent image deterioration due to gap variationbetween the jetting head and the recording medium, each roller pressingthe recording medium is preferably arranged as close to an end of thehead chip as possible in the vicinity of boundary nozzles causingdensity unevenness easily. However, if positions of the rollers in thenozzle arrangement direction are the same between the jetting heads byarranging each roller close to an end of the head chip for all of thejetting heads, each of the rollers sequentially presses the sameposition of the recording medium, which results in conspicuous rollermarks on the recording medium.

Thus, it is desired that the boundary nozzles of the head chips and therollers be optimally arranged in the two jetting heads to reduce rollermarks and density unevenness in an image part for which printing isperformed with the boundary nozzles.

An object of the present teaching is to prevent, in a configuration inwhich head chip groups are arranged to partially face each other in aconveyance direction of a recording medium, overlap of roller marks andoverlap of density unevenness due to boundary nozzles of the head chipgroups.

According to an aspect of the present teaching, there is provided aprinter configured to perform printing on a recording medium, including:

-   -   first head chips corresponding to a first ink and arrayed at        intervals in a second direction orthogonal to a first direction        in which the recording medium is conveyed;    -   second head chips corresponding to the first ink and arrayed at        intervals in the second direction at positions which are        different from arrangement positions of the first head chips in        the second direction and are adjacent to arrangement positions        of the first head chips in the first direction;    -   third head chips corresponding to a second ink and arrayed at        intervals in the second direction at positions respectively        corresponding to the arrangement positions of the first head        chips in the second direction;    -   fourth head chips corresponding to the second ink and arrayed at        intervals in the second direction at positions which        respectively correspond to arrangement positions of the second        head chips in the second direction and are adjacent to        arrangement positions of the third head chips in the first        direction;    -   a first roller arranged to face one of the first head chips in        the first direction on one side of a second head chip N in the        second direction, the second head chip N being a N-th second        head chip of the second head chips counted from the one side in        the second direction; and    -   a second roller arranged to face one of the third head chips in        the first direction on one side of a fourth head chip N in the        second direction, the fourth head chip N being an N-th fourth        head chip of the fourth head chips counted from the one side in        the second direction,    -   wherein, when it is assumed that a nozzle, of nozzles arranged        in the second head chip N and used for printing, which is        positioned closest to the one side in the second direction is        referred to as a nozzle A and that a nozzle, of nozzles arranged        in the fourth head chip N and used for printing, which is        positioned closest to the one side in the second direction is        referred to as a nozzle B, the nozzle A is arranged closer to        the one side in the second direction than the nozzle B and the        first roller is arranged closer to the other side in the second        direction than the second roller.

In the present teaching, the position, of the first roller provided fora head chip group (first and second head chips) corresponding to thefirst ink, in the second direction is different from the position, ofthe second roller provided for a head chip group (third and fourthchips) corresponding to the second ink, in the second direction. Thisreduces roller marks in the recording medium.

In typical head chips, an end nozzle of one of the head chips to be usedfor printing has density unevenness easily due to ink landing deviationsbetween ink droplets from the end nozzle and ink droplets from nozzlesof another head chip that partially faces the head chip having the endnozzle in a conveyance direction. In the present teaching, however, thenozzle A, of nozzles arranged in the second head chip N and used forprinting, which is positioned closest to the one side in the seconddirection and the nozzle B, of nozzles arranged in the fourth head chipN and used for printing, which is positioned closest to the one side inthe second direction have mutually different positions in the seconddirection. Thus, the present teaching prevents deterioration of imagequality which would be otherwise caused by overlap of image densityunevenness due to the end nozzles of different head chip groups.

The nozzle A arranged at the end of the second head chip N and used forprinting is positioned closer to the end side in the second directionthan the nozzle B arranged at the end of the fourth head chip N and usedfor printing. Further, the first roller corresponding to the second headchip N is arranged close to the nozzle A. Thus, the distance between thenozzle A arranged at the end of the second head chip N and used forprinting and the first roller is shorter than the distance between thenozzle B arranged at the end of the fourth head chip N and used forprinting and the second roller.

In a typical boundary nozzle group which may cause density unevenness,if a gap between each head chip and the recording medium varies, thedensity unevenness could be more conspicuous. Thus, a roller pressingthe recording medium is preferably arranged close to the boundary nozzlegroup. In a configuration in which the head chip groups have mutuallydifferent positions of the boundary nozzle groups and mutually differentpositions of the rollers, however, it may be difficult to arrange therollers close to the boundary nozzle groups. Thus, in the presentteaching, the first roller is arranged close to the boundary nozzlegroup in each of the first and second head chips corresponding to thefirst ink, and the second roller is arranged distant from the boundarynozzle group in each of the third and fourth head chips corresponding tothe second ink.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a printer according to an embodimentof the present teaching.

FIG. 2 is a plan view of four ink-jet heads.

FIG. 3 is a plan view of one of the ink-jet heads and a roller unit.

FIG. 4 is an enlarged view of two head chips of one of the ink-jetheads.

FIG. 5 is a graph indicating a nozzle use-ratio between two head chips.

FIG. 6 is a cross-sectional view taken along a line VI-VI in FIG. 3.

FIG. 7 is an enlarged view of FIG. 2.

FIG. 8 is a plan view of another arrangement of the four ink-jet heads.

FIG. 9 is a schematic plan view of two ink-jet heads and roller unitsaccording to a second modified embodiment.

FIG. 10 is an enlarged plan view of four ink-jet heads according to thesecond modified embodiment.

DESCRIPTION OF THE EMBODIMENTS

Subsequently, an embodiment of the present teaching will be described. Aconveyance direction in which a recording sheet 100 is conveyed in FIG.1 is defined as a front-rear direction of a printer 1. A width directionof the recording sheet 100 (hereinafter also referred to as “sheet widthdirection”) is defined as a left-right direction of the printer 1. Adirection perpendicular to a paper surface of FIG. 1, i.e.,perpendicular to the front-direction and left-right direction, isdefined as an up-down direction of the printer 1.

<Schematic Configuration of Printer>

As depicted in FIG. 1, the printer 1 includes a platen 3, four ink-jetheads 4, two conveyance rollers 5 and 6, a controller 7, a roller unit8, and the like, those of which are contained in a housing 2 of theprinter 1.

The recording sheet 100 is placed on an upper surface of the platen 3.The four ink-jet heads 4 are arranged in the conveyance direction abovethe platen 3. Inks are supplied from unillustrated ink tanks to therespective ink-jet heads 4, and any of four color inks (black, yellow,cyan, and magenta inks) is supplied to the corresponding one of the fourink-jet heads 4. Namely, the four ink-jet heads 4 jet inks havingmutually different colors, respectively.

In the following description, configurations corresponding to black,magenta, yellow, and cyan are assigned with alphabetic suffixes of “k”indicating black, “m” indicating magenta, “y” indicating yellow, and “c”indicating cyan, respectively. For example, an ink-jet head 4 k depictedin FIG. 1 represents the ink-jet head 4 jetting black ink.

The controller 7 includes a Central Processing Unit (CPU), a Read OnlyMemory (ROM), a Random Access Memory (RAM), and an Application SpecificIntegrated Circuit (ASIC) including various control circuits. Thecontroller 7 includes a nonvolatile memory storing various controlparameters in a rewritable manner. The controller 7 is connected in datacommunication with an external apparatus 9, such as a PC, to controlrespective parts of the printer 1, such as the four ink-jet heads 4 anda conveyance motor, based on image data sent from the external apparatus9.

More specifically, the controller 7 controls the conveyance motordriving the conveyance rollers 5 and 6 to convey the recording sheet 100by use of the two conveyance rollers 5 and 6 in the conveyancedirection. In parallel with the sheet conveyance, the controller 7controls the four ink-jet heads 4 to jet inks to the recording sheet100. Accordingly, an image is printed on the recording sheet 100.

<Ink-Jet Heads and Roller Unit>

As depicted in FIGS. 2, 3, and 6, the four ink-jet heads 4 and a rollerunit 8 are arranged above the platen 3. The roller unit 8 has fouropenings 36 corresponding to the four ink-jet heads 4, respectively. Theink-jet heads 4 are arranged on the roller unit 8. The ink-jet heads 4may move to separate from the roller unit 8. The ink-jet heads 4 move topositions separate from the roller unit 8 in a case of maintenance, suchas a purge.

<Ink-Jet Heads>

The ink-jet heads 4 will be described first. As depicted in FIG. 2, eachof the ink-jet heads 4 includes two head chips 11 arranged with aninterval in the left-right direction and two head chips 12 arranged withan interval in the left-right direction. The head chips 11 and 12 aremounted on a chip holding plate 10.

As depicted in FIGS. 3 and 6, notches 16 are formed at parts, of thechip holding plate 10, including no head chips 11 and no head chips 12.Each of the notches 16 is formed by a side surface 13 orthogonal to thefront-rear direction, a side surface 14 orthogonal to the left-rightdirection, and a side surface 15 perpendicular to the up-down direction.Each of the notches 16 accommodates a roller 33 and the like of theroller unit 8.

The head chips 11 and 12 are alternately arranged in the left-rightdirection in a state of deviating from each other in the conveyancedirection. The head chips 11 are arranged on the rear side and the headchips 12 are arranged on the front side in the conveyance direction, asdepicted in FIG. 2. Namely, the head chips 11 and 12 are alternatelyarranged in the left-right direction to form a zigzag shape in which thehead chips 11 are arranged on the rear side and the head chips 12 arearranged on the front side in the conveyance direction. Each of the headchips 11 includes nozzles 21 arrayed in the left-right direction, andeach of the head chips 12 includes nozzles 22 arrayed in the left-rightdirection. In the present embodiment, the positions of the head chips 11and 12 in the left-right direction are the same between the ink-jetheads 4 for the respective ink colors.

An area of the head chip 11 formed with the nozzles 21 and an area, ofthe head chip 12 adjacent to the head chip 11, formed with the nozzles22 are positioned to partially face each other in the front-reardirection.

In FIG. 3, a right end area of the leftmost head chip 11 surrounded by achain line where the nozzles 21 are arranged to face some of the nozzles22 of the head chip 12 in the front-rear direction is defined as an areaA. Further, a left end area, of the head chip 12 arranged immediately onthe right of the leftmost head chip 11, surrounded by a chain line wherethe nozzles 22 are arranged to face the nozzles 21 of the area A of thehead chip 11 in the front-rear direction is defined as an area B. Theposition of the area A is coincident with the position of the area B inthe left-right direction, and the position of the nozzles 21 in the areaA is coincident with the position of the nozzles 22 in the area B in theleft-right direction.

Similarly, regarding any other head chips 11 and 12 arranged adjacent toeach other, the head chips 11 and 12 are arranged to partially face eachother in the front-rear direction. Arranging the head chips 11 and 12alternately in the left-right direction in a state of partially facingeach other in the front-rear direction forms a single line head in whichthe nozzles 21 and the nozzle 22 are arranged at regular intervals inthe left-right direction.

The controller 7 performs the following jetting control for each of theink-jet heads 4 configured as described above. For sake of simplicity,the area A of the leftmost head chip 11 and the area B of the head chip12 arranged immediately on the right of the leftmost head chip 11 willbe explained. The same is true on any other head chips 11 and 12.

In the area A of the head chip 11 and the area B of the head chip 12,there are nozzles 21 and 22 configured to be jet ink on the recordingsheet 100 from both of the head chip 11 and the head chip 12. Thejetting control in the areas A and B will be explained in detail withreference to FIGS. 4 and 5.

In FIG. 4, chain lines divide the area A of the head chip 11 into anarea A1 from which ink is jetted, an area AX from which ink is jetted,and an area A2 from which no ink is jetted. The hatched area in FIG. 4is the area AX. Further, chain lines divide the area B of the head chip12 into an area B1 from which no ink is jetted, an area BX from whichink is jetted, and an area B2 from which ink is jetted. The hatched areain FIG. 4 is the area BX.

In the area AX and the area BX, ink is jetted from both of the nozzles21 and 22. The position of the area A1 is coincident with the positionof the area B1 in the left-right direction. The same is true on theareas AX and BX and the areas A2 and B2.

In the following, the area AX of the head chip 11 is referred to as aboundary area AX, and the area BX of the head chip 12 is referred to asa boundary area BX. The nozzles 21 arranged in the area AX are referredto as boundary nozzles 21X and the nozzles 22 arranged in the area BXare referred to as boundary nozzles 22X. Information about the positionsof the boundary nozzles 21X and 22X is stored in the ROM or thenonvolatile memory in the controller.

For sake of simplicity, in FIG. 4, from among the boundary nozzles 21Xand 22X, the leftmost boundary nozzle 21X and the leftmost boundarynozzle 22X are referred to as a boundary nozzle 21L and a boundarynozzle 22L, respectively, and the rightmost boundary nozzle 21X and therightmost boundary nozzle 22X are referred to as a boundary nozzle 21Rand a boundary nozzle 22R, respectively.

FIG. 5 depicts changes in a use-ratio R1 of the nozzles 21 of the headchip 11 and a use-ratio R2 of the nozzles 22 of the head chip 12depending on positions in the left-right direction. In this context, theuse-ratio means a ratio of dots to be formed by nozzles of one of thehead chips for dots to be formed in a predefined area of the recordingmedium.

For example, when 10 dots are formed in an area based on density data ofeach color ink that is obtained by image processing for image dataincluding RGB data or the like, the nozzle use-ratio of the head chip 11in that area may be 70%. In that case, seven dots of 10 dots are formedby the nozzles 21 of the head chip 11 and remaining three dots areformed by the nozzles 22 of the head chip 12.

In the above case, 0<R1<100% is satisfied in the position of theboundary nozzles 21X, and 0<R2<100% is satisfied in the position of theboundary nozzles 22X. The nozzles 21 arranged on the right of theboundary nozzle 21R are not used for printing, and the nozzles 22arranged on the left of the boundary nozzle 22L are not used forprinting.

Even when the above-described control is performed, an image part formedby the boundary nozzles 21X and 22X still has density unevenness, ascompared to an image part formed by only the nozzles of a single headchip. Thus, when the positions of the boundary nozzles 21X and 22X inthe left-right direction are the same between the four ink-jet heads 4,density unevenness caused by inks jetted from the boundary nozzles 21Xand 22X of the respective four ink-jet heads 4 may overlap with eachother, resulting in an increase in the density unevenness.

Thus, as depicted in FIG. 7, the four ink-jet head 4 have mutuallydifferent positions of the boundary areas AX of the areas A and theboundary areas BX of the areas B. In the following, the positions, inthe left-right direction, of the leftmost boundary area AX and theleftmost boundary area BX will be described specifically. As depicted inFIG. 7, a two-dot chain line that represents a center position in theleft-right direction and is common between the areas A and B is referredto as a center line C.

A boundary area AXk of the head chip 11 and a boundary area BXk of thehead chip 12 for black ink are positioned on the left of the center lineC. Similarly, a boundary area AXy of the head chip 11 and a boundaryarea BXy of the head chip 12 for yellow ink are positioned on the leftof the center line C. The boundary areas AXk and BXk are positioned onthe left of the boundary areas AXy and BXy. Namely, a boundary nozzle22Lk arranged on the leftmost position of the boundary nozzles 22Xk ofthe head chip 12 k is positioned on the left of a boundary nozzle 22Lyarranged on the leftmost position of the boundary nozzles 22Xy of thehead chip 12 y.

Meanwhile, boundary areas AXm and BXm for magenta ink and boundary areasAXc and BXc for cyan ink are positioned on the right of the center lineC. The boundary areas AXm and BXm are positioned on the right of theboundary areas AXc and BXc.

The distance between the center line C and the boundary area AXk forblack ink is equal to the distance between the center line C and theboundary area AXm for magenta ink. The same is true on the boundary areaBXk and the boundary area BXm. Further, the distance between the centerline C and the boundary area AXy for yellow ink is equal to the distancebetween the center line C and the boundary area AXc for cyan ink. Thesame is true on the boundary area BXy and the boundary area BXc.

The boundary areas AXk and BXk and the boundary areas AXm and BXm aresymmetrically arranged across the center line C, and the boundary areasAXy and BXy and the boundary areas AXc and BXc are symmetricallyarranged across the center line C.

Although only the leftmost boundary areas AX and BX have been explainedabove, the same is true on other boundary areas.

<Roller Unit>

Subsequently, the roller unit 8 will be explained. As depicted in FIGS.3 and 6, the roller unit 8 includes a frame 31 fixed to a body of thehousing 2, and rollers 33, 34, and 35 attached to the frame 31.

The frame 31, of which outer circumference is a substantiallyrectangular shape as view from above, is long in the left-rightdirection and fixed to the housing 2. The frame 31 has four rectangularopenings 36, and each of the ink-jet heads 4 is inserted into thecorresponding one of the openings 36 from above.

Each of the rollers 33 to 35 has a gear shape and is rotatably supportedby a support part 32. Each of the ink-jet heads 4 includes three rollers33.

The rollers 33 to 35 are accommodated in the notches 16, each of whichis formed by the above-described side surfaces 13, 14, and 15 and isprovided at a lower part of the chip holding plate 10, in a state wherethe ink-jet heads 4 are inserted into the openings 36, respectively. Therollers 33 to 35 do not interfere with the ink-jet heads 4.

As depicted in FIG. 6, the rollers 33 to 35 press the recording sheet100 being conveyed on the platen 3 from above. This prevents variationin a gap G between an ink jetting surface 17 of each head chip 11 or thelike and the recording sheet 100, thus reducing an ink landingdeviation.

As depicted in FIGS. 2 and 7, the four ink-jet heads 4 k, 4 m, 4 y, and4 c corresponding to the inks of four colors are arranged. In thepresent embodiment, the four ink-jet heads are arranged in the order ofblack, magenta, yellow, and cyan from the rear side to the front side,i.e., in the order of KMYC.

<Layout for Roller>

Subsequently, a layout for the rollers 33 to 35 will be explained. Atfirst, the rollers 33 arranged in the vicinities of the areas AX and BXof the head chips 11 and 12 will be explained. Although each of theink-jet heads 4 includes the three rollers 33, a relation between theroller 33 and the head chip 11 adjacent to the roller 33 and a relationbetween the roller 33 and the head chip 12 adjacent to the roller 33 arecommon between the three rollers 33. Thus, the layout for the leftmostroller 33 will be explained as a representative.

The roller 33 k for black ink is arranged on the left of the head chip12 k at a position facing the head chip 11 k in the front-reardirection. Namely, the roller 33 k is arranged on the left of theboundary areas AX and BX.

As with the roller 33 k, the roller 33 y for yellow ink is arranged onthe left of the head chip 12 y at a position facing the head chip 11 yin the front-rear direction. Namely, the roller 33 y is arranged on theleft of the areas AX and BX. The roller 33 y is arranged on the left ofthe roller 33 k. Thus, the distance between the roller 33 k and the headchip 12 k adjacent to the roller 33 k is smaller than the distancebetween the roller 33 y and the head chip 12 y adjacent to the roller 33y.

The roller 33 m for magenta ink is arranged on the right of the headchip 11 m at a position facing the head chip 12 m in the front-reardirection. Namely, the roller 33 m is arranged on the right of theboundary areas AX and BX. The roller 33 m and the roller 33 k aresymmetrically arranged across the center line C for the areas A and B.

The roller 33 c for cyan ink is arranged similarly as the roller 33 m.The roller 33 c is arranged on the right of the roller 33 m. The roller33 c and the roller 33 y are symmetrically arranged across the centerline C for the areas A and B.

Subsequently, the roller 34 will be explained. As depicted in FIG. 3,the roller 34 is accommodated in a notch 16, of the notches 16 of thechip holding plate 10, including no roller 33. The roller 34 is providedto press the recording sheet 100 more reliably.

As depicted in FIG. 2, the roller 34 k for black ink is arranged on theright of the rightmost head chip 11 k at a position facing the head chip12 k in the front-rear direction. Namely, the roller 34 k is arranged onthe right of the rightmost roller 33 k to press the vicinity of a rightend of the recording sheet 100.

Similarly, the roller 34 y for yellow ink is arranged on the right ofthe rightmost head chip 11 y at a position facing the head chip 12 y inthe front-rear direction.

The roller 34 m for magenta ink is arranged on the left of the leftmosthead chip 12 m at a position facing the head chip 11 m in the front-reardirection. The roller 34 m presses the vicinity of a left end of therecording sheet 100. The roller 34 c for cyan ink is arranged similarly.

The distance between the roller 34 k and the rightmost head chip 11 k inthe left-right direction is longer than the distance between the roller33 k and the head chip 12 k adjacent to the roller 33 k in theleft-right direction. Thus, the roller 34 k presses a position closer tothe vicinity of the right end of the recording sheet 100 than the roller33 k. Similarly, the distance between the roller 34 y and the rightmosthead chip 11 y in the left-right direction is longer than the distancebetween the roller 33 y and the head chip 12 k adjacent to the roller 33y in the left-right direction.

The distance between the roller 34 m and the leftmost head chip 12 m inthe left-right direction is longer than the distance between the roller33 m and the head chip 11 m adjacent to the roller 33 m in theleft-right direction. Thus, the roller 34 m presses a position closer tothe vicinity of the left end of the recording sheet 100 than the roller33 m. The roller 34 c is arranged similarly to the roller 34 m.

The ink-jet heads 4 are arranged in the order of KMYC in the front-reardirection, and thus the rollers 34 are alternately arranged at the rightends and left ends of the ink-jet heads 4. This allows the rollers 34 topress the right end and left end of the recording sheet 100 in abalanced manner.

Subsequently, rollers 35 will be explained. A roller 35 k for black inkis arranged on the left of the leftmost roller 33 k at a position facingthe leftmost head chip 11 k in the front-rear direction. Namely, theroller 35 k is arranged on the outside of the leftmost roller 33 k inthe left-right direction to press the vicinity of the left end of therecording sheet 100.

Similarly, a roller 35 y for yellow ink is arranged on the left of theleftmost roller 33 y at a position facing the leftmost head chip 11 y inthe front-rear direction.

A roller 35 m for magenta ink is arranged on the right of the rightmostroller 33 m at a position facing the rightmost head chip 12 m in thefront-rear direction. Namely, the roller 35 m is arranged on the outsideof the rightmost roller 33 m in the left-right direction to press thevicinity of the right end of the recording sheet 100. A roller 35 c forcyan ink is similarly arranged.

The position of the roller 35 k in the left-right direction is differentfrom the position of the roller 35 y in the left-right direction. Thesame is true on the roller 35 m and the roller 35 c. Accordingly, rollermarks are reduced.

<Positional Relation Between Roller and Boundary Nozzles>

Subsequently, a positional relation between the roller 33 and theboundary areas AX and BX of the two head chips 11 and 12 will bedescribed. In the present embodiment, the ink-jet head 4 for black ink,from among the inks of four colors including black, yellow, cyan, andmagenta, having conspicuous density unevenness easily is configured suchthat the roller 33 is positioned close to the boundary areas AX and BX.

In particular, the distance between the roller 33 for each of the blackand magenta inks and the boundary nozzles 21X and 22X is preferentiallyreduced. Since black and magenta inks jetted from the boundary nozzles21X and 22X easily have conspicuous density unevenness, gap variationduring printing is required to be reduced by providing the rollers 33for black and magenta inks at positions closer to the boundary nozzles21X and 22X. Yellow and cyan inks are not likely to have conspicuousdensity unevenness, and thus the distance between the roller 33 for eachof the yellow and cyan inks and the boundary nozzles 21X and 22X may berelatively long.

On the basis of the above, the positional relation between the roller 33and the boundary nozzles 21X and 22X will be described. At first, theroller 33 k and the boundary nozzles 21Xk and 22Xk for black ink are onthe left of the center line C for the areas A and B. The distance, inthe left-right direction, between the roller 33 k and the leftmostboundary nozzle 22 LK of the boundary nozzles 22Xk is referred to as adistance Dk.

The roller 33 y and the boundary nozzles 21Xy and 22Xy for yellow inkare positioned on the left of the center line C. The distance, in theleft-right direction, between the roller 33 y and the leftmost boundarynozzle 22Ly of the boundary nozzles 22Xy is referred to as a distanceDy. As described above, the roller 33 k is arranged on the right of theroller 33 y, and the boundary nozzle 22Lk is arranged on the left of theboundary nozzle 22Ly. Thus, the distance Dk is shorter than the distanceDy.

In the present embodiment, since the density unevenness of black ink ismore conspicuous than the density unevenness of yellow ink, the distanceDk is preferentially reduced. The density unevenness of yellow ink isnot likely to be conspicuous, and thus making the distance Dy slightlylong hardly affects image quality.

The roller 33 m and the boundary nozzles 21Xm and 22Xm for magenta inkare positioned on the right of the center line C. The distance, in theleft-right direction, between the roller 33 m and the rightmost boundarynozzle 21Rm of the boundary nozzles 21Xm is referred to as a distanceDm. As described above, the rollers 33 k and the rollers 33 m aresymmetrically arranged across the center line C, and the same is true onthe boundary areas AXk, BXk and the boundary areas AXm, BXm. Thus, thedistance Dk is equal to the distance Dm. Namely, the distance Dm isshorter than the distance Dy.

The roller 33 c and the boundary nozzles 21Xc and 22Xc for cyan ink arepositioned on the right of the center line C. The distance, in theleft-right direction, between the roller 33 c and the rightmost boundarynozzle 21Rc of the boundary nozzles 21Xc is referred to as a distanceDc. As described above, the roller 33 m is arranged on the left of theroller 33 c, and the boundary nozzle 21Xm is arranged on the right ofthe boundary nozzle 21Xc. Thus, the distance Dm is shorter than thedistance Dc. As with the case of black and yellow inks, since thedensity unevenness of magenta ink is more conspicuous than the densityunevenness of cyan ink, the distance Dm is preferentially reduced.

In the above configuration, the positions of the rollers 33 in theleft-right direction are different between the ink-jet heads 4 for therespective four color inks. Thus, roller marks are made at mutuallydifferent positions between the ink-jet heads 4, which makes the rollermarks inconspicuous. Further, in the above configuration, the positionsof the boundary nozzles 21X and 22X in the left-right direction aredifferent between the ink-jet heads 4 for the respective four colorinks. Thus, density unevenness occurs at various positions, whichprevents deterioration of image quality.

The variation in the gap G may be reduced by arranging each roller 33 asclose to the boundary nozzles 21X and 22X as possible. However, it isdifficult to reduce the distance between each roller 33 and the boundarynozzles 21X and 22X for all of the ink-jet heads 4. Thus, in the presentembodiment, black ink of which density unevenness is most likely to beconspicuous has a reduced distance.

The roller 33 may be arranged at a position that is symmetrical with theroller 33 k for black ink in the left-right direction about the centerline C for the areas A and B. In the present embodiment, the roller 33 mfor magenta ink that may have the second most conspicuous densityunevenness is arranged at a position close to the boundary nozzles 21Xmand 22Xm.

The distance Dk for black ink may be equal to the distance Dm formagenta ink, provided that each roller 33 is arranged as close to theboundary nozzles 21X and 22X as possible.

Yellow ink is not likely to have conspicuous density unevenness. Thus,even when the roller 33 y is slightly distant from the boundary nozzles21Xy and 22Xy to cause variation in the gap G, the effect on imagequality is smaller than those of the black and magenta inks. The same istrue on cyan ink.

The rollers 34 are arranged in the notches 16 in which no rollers 33 areaccommodated, and thus deterioration in image quality caused by gapvariation during printing is further prevented.

The arrangement order of the ink-jet heads 4 allows the rollers 34 to bearranged zigzag, and thus gap variation is further effectively preventedin the vicinities of ends of the recording sheet 100 in the left-rightdirection.

The rollers 35 are arranged on the outsides of the rollers 33, and thusgap variation is further effectively prevented in the vicinities of endsof the recording sheet 100 in the left-right direction.

In the above-described embodiment, the front-rear direction correspondsto “first direction” of the present teaching; the left-right directioncorresponds to “second direction” of the present teaching; the recordingsheet 100 corresponds to “recording medium” of the present teaching; thehead chip 11 k corresponds to “first head chip” of the present teaching;the head chip 12 k corresponds to “second head chip” of the presentteaching; the head chip 11 y corresponds to “third head chip” of thepresent teaching; the head chip 12 y corresponds to “fourth head chip”of the present teaching; the head chip 11 m corresponds to “fifth headchip” of the present teaching; and the head chip 12 m corresponds to“sixth head chip” of the present teaching.

The roller 33 k corresponds to “first roller” of the present teaching;the roller 33 y corresponds to “second roller” of the present teaching;the roller 33 m corresponds to “third roller” of the present teaching;the roller 34 k corresponds to “first auxiliary roller” of the presentteaching; the roller 34 y corresponds to “second auxiliary roller” ofthe present teaching; the roller 34 m corresponds to “third auxiliaryroller” of the present teaching; the roller 35 k corresponds to “fourthauxiliary roller” of the present teaching; the roller 35 y correspondsto “fifth auxiliary roller” of the present teaching; the boundary nozzle22Lk corresponds to “nozzle A” of the present teaching; and the boundarynozzle 22Ly corresponds to “nozzle B” of the present teaching.

In the embodiment, the positions of the head chips 11 and 12 in theleft-right direction are the same between the ink-jet heads 4. Thepresent teaching, however, is not limited thereto.

In the embodiment, each of the ink-jet heads 4 includes two head chips11 and two head chips 12, the number of head chips 11 and 12 is notlimited two, and each of the ink-jet heads 4 may include any number ofthe head chips 11 and 12.

Instead of the rollers 33, 34, and 35, for example, circular rubberrollers having a smooth outer circumferential surface may be used,provided that they may function as rollers pressing the recording sheet100. Further, instead of the rollers, the structure by which therecording sheet 100 is pressed may be a structure, such as a protrusion,protruding on a platen side beyond the nozzle surface and having asmooth surface in which an end that may make contact with the recordingsheet is chamfered.

Subsequently, modified embodiments in which modifications are added tothe embodiment will be described. The components or parts which are thesame as those of the above embodiment are designated by the samereference numerals, and any explanation thereof will be omitted asappropriate.

First Modified Embodiment

As depicted in FIG. 8, the ink-jet heads 4 are arranged in the order ofKYMC from the rear side to the front side. In such a configuration, thepositions of the rollers 33 in the left-right direction are differentfrom each other, and thus the recording sheet 100 may have inconspicuousroller marks. Further, the positions of the boundary areas AX and BX inthe left-right direction are different between the ink-jet heads 4, andthus an image to be formed may have inconspicuous density unevenness.The positional relations between the rollers 33 and the boundary nozzles22X and 22X in the left-right direction for respective inks according tothe first modified embodiment are the same as those of the aboveembodiment, the ink easily having conspicuous density unevenness mayhave a preferentially reduced distance between each roller 33 and theboundary nozzles 22X and 22X in the left-right direction.

Second Modified Embodiment

The distance between the roller 33 and one end of the head chip 11 orhead chip 12 in the left-right direction is preferably the same as thedistance between the roller 33 and the other end of the head chip 11 orhead chip 12 in the left-right direction. Such a configuration, however,may not be obtained in some cases.

For example, as depicted in FIG. 9, each of the chip holding plates 10 aincludes a notch 16 and three notches 16 a. Only the notch 16 arrangedon the leftmost side is formed in a substantially rectangularparallelepiped space. Each of the notches 16 a has an inclined surfacerather than the substantially rectangular parallelepiped shape. Namely,unlike each side surface 14 orthogonal to the left-right direction, eachside surface 14 a forming the corresponding notch 16 a is inclined inthe left-right direction. Such a configuration may be adopted to improvemaintainability as described, for example, in Japanese PatentApplication laid open No. 2015-231721.

In the above case, the distance between the right end of the leftmosthead chip 11 and the inclined side surface 14 a is longer than a case inwhich the side surface is not inclined. Thus, it is difficult for theabove case to make the distance between the roller 33 m and the boundarynozzle 21Rm for magenta ink and the distance between the roller 33 k andthe boundary nozzle 22Lk for black ink equal.

Even when the side surface 14 a is not inclined, the roller 33 m mayhave difficulty in being provided close to the head chip 11 m for somereasons, for example, a temperature sensor needs to be attached to theside surface 14 a.

Thus, as depicted in FIG. 10, the roller 33 m for magenta ink isarranged at a position that is on the right of the position of theroller 33 m in the above embodiment. Namely, the distance between thehead chip 21 m and the roller 33 in the left-right direction is greaterthan the distance between the head chip 22 k and the roller 33 k.Further, the roller 33 c for cyan ink is arranged at position that is onthe right of the position of the roller 33 c in the above embodiment,and the position of the roller 33 c is different from the position ofthe roller 33 m in the left-right direction. The distance between thehead chip 21 m and the roller 33 m in the left-right direction issmaller than the distance between the head chip 22 y and the roller 33 yfor yellow ink.

Thus, a distance Dma between the roller 33 m and the boundary nozzle21Rm is greater than the distance Dk between the roller 33 k and theboundary nozzle 22Lk. In that configuration, making the distance Dmalonger is allowed to prevent density unevenness of black inkpreferentially. Similarly, making a distance Dca between the roller 33 cand the boundary nozzle 21Rc longer is allowed. Density unevenness ofmagenta ink, however, is more conspicuous than yellow and cyan inks, andthus it needs to be reduced. In view of this, the distance Dma issmaller than the distance Dca and the distance Dy between the roller 33y and the boundary nozzle 22Ly.

Third Modified Embodiment

In areas AX and BX of head chips 11 and 12 of a third modifiedembodiment, ink is not jetted from both of the head chips 11 and 12.Instead, use nozzles of the head chips 11 and 12 are used selectively atboundaries in the left-right direction. In that case, no boundarynozzles 21X and 22X are present. The rightmost nozzle 21 of nozzles 21to be used for printing is the boundary nozzle 21R and the leftmostnozzle 22 of nozzles 22 to be used for printing is the boundary nozzle22L.

Fourth Modified Embodiments

In the above embodiment and modified embodiments, magenta, yellow, andcyan inks jetted from the respective ink-jet heads 4 may be replacedwith each other. Inks having any other colors than black, magenta,yellow, and cyan may be used. The number of the ink-jet heads 4 is notlimited to four. For example, three ink-jet head heads 4 may be providedto jet magenta, yellow, and cyan inks respectively.

What is claimed is:
 1. A printer configured to perform printing on arecording medium, comprising: first head chips corresponding to a firstink and arrayed at intervals in a second direction orthogonal to a firstdirection in which the recording medium is conveyed; second head chipscorresponding to the first ink and arrayed at intervals in the seconddirection at positions which are different from arrangement positions ofthe first head chips in the second direction and are adjacent toarrangement positions of the first head chips in the first direction;third head chips corresponding to a second ink and arrayed at intervalsin the second direction at positions respectively corresponding to thearrangement positions of the first head chips in the second direction;fourth head chips corresponding to the second ink and arrayed atintervals in the second direction at positions which respectivelycorrespond to arrangement positions of the second head chips in thesecond direction and are adjacent to arrangement positions of the thirdhead chips in the first direction; a first roller arranged to face oneof the first head chips in the first direction on one side of a secondhead chip N in the second direction, the second head chip N being a N-thsecond head chip of the second head chips counted from the one side inthe second direction; and a second roller arranged to face one of thethird head chips in the first direction on one side of a fourth headchip N in the second direction, the fourth head chip N being an N-thfourth head chip of the fourth head chips counted from the one side inthe second direction, wherein, when it is assumed that a nozzle, ofnozzles arranged in the second head chip N and used for printing, whichis positioned closest to the one side in the second direction isreferred to as a nozzle A and that a nozzle, of nozzles arranged in thefourth head chip N and used for printing, which is positioned closest tothe one side in the second direction is referred to as a nozzle B, thenozzle A is arranged closer to the one side in the second direction thanthe nozzle B and the first roller is arranged closer to the other sidein the second direction than the second roller.
 2. The printer accordingto claim 1, further comprising: fifth head chips corresponding to athird ink and arrayed at intervals in the second direction at positionsrespectively corresponding to the arrangement positions of the firsthead chips in the second direction; sixth head chips corresponding tothe third ink and arrayed at intervals in the second direction atpositions which respectively correspond to the arrangement positions ofthe second head chips in the second direction and are adjacent toarrangement positions of the fifth head chips in the first direction;and a third roller arranged to face one of the sixth head chips in thefirst direction on the other side of a fifth head chip N in the seconddirection, the fifth head chip N being an N-th fifth head chip of thefifth head chips counted from the one side in the second direction,wherein a distance between the fifth head chip N and the third roller inthe second direction is shorter than a distance between the fourth headchip N and the second roller in the second direction.
 3. The printeraccording to claim 2, wherein a distance between the second head chip Nand the first roller in the second direction is identical to thedistance between the fifth head chip N and the third roller in thesecond direction.
 4. The printer according to claim 2, wherein thedistance between the fifth head chip N and the third roller in thesecond direction is longer than a distance between the second head chipN and the first roller in the second direction.
 5. The printer accordingto claim 2, further comprising: a first auxiliary roller arranged toface one of the second head chips in the first direction at a positioncloser to the other side in the second direction than a first head chipE which is a first head chip, of the first head chips, arranged closestto the other side in the second direction; a second auxiliary rollerarranged to face one of the fourth head chips in the first direction ata position closer to the other side in the second direction than a thirdhead chip E which is a third head chip, of the third head chips,arranged closest to the other side in the second direction; and a thirdauxiliary roller arranged to face one of the fifth head chips in thefirst direction at a position closer to the one side in the seconddirection than a sixth head chip E which is a sixth head chip, of thesixth head chips, arranged closest to the one side in the seconddirection, wherein a distance between the second head chip N and thefirst roller in the second direction is shorter than a distance betweenthe first head chip E and the first auxiliary roller in the seconddirection, the distance between the fourth head chip N and the secondroller in the second direction is shorter than a distance between thethird head chip E and the second auxiliary roller in the seconddirection, and the distance between the fifth head chip N and the thirdroller in the second direction is shorter than a distance between thesixth head chip E and the third auxiliary roller in the seconddirection.
 6. The printer according to claim 5, wherein the fifth headchips and the sixth head chips are arranged between the first and secondhead chips and the third and fourth head chips in the first direction,and the third auxiliary roller is arranged between the first auxiliaryroller and the second auxiliary roller in the first direction.
 7. Theprinter according to claim 1, further comprising: a fourth auxiliaryroller arranged to face one of the first head chips in the firstdirection at a position closer to the one side in the second directionthan a first roller, of the first rollers, arranged closest to the oneside in the second direction; and a fifth auxiliary roller arranged toface one of the third head chips in the first direction at a positioncloser to the one side in the second direction than a second roller, ofthe second rollers, arranged closest to the one side in the seconddirection; wherein the position of the fourth auxiliary roller in thesecond direction is different from the position of the fifth auxiliaryroller in the second direction.
 8. The printer according to claim 1,wherein the first ink is a black ink and the second ink is one of amagenta ink, a yellow ink, and a cyan ink.
 9. The printer according toclaim 1, wherein the first ink is a magenta ink and the second ink is ayellow ink or a cyan ink.
 10. The printer according to claim 2, whereinthe first ink is a black ink, the second ink is a yellow ink or a cyanink, and the third ink is a magenta ink.
 11. A printer configured toperform printing on a recording medium, comprising: a first head chipcorresponding to a first ink and formed with nozzles which are arrayedin a second direction orthogonal to a first direction in which therecording medium is conveyed; a second head chip corresponding to thefirst ink, formed with nozzles arrayed in the second direction, andarranged at a position which is different from an arrangement positionof the first head chip in the second direction and is adjacent to anarrangement position of the first head chip in the first direction; athird head chip corresponding to a second ink, formed with nozzlesarrayed in the second direction, and arranged at a position whichcorresponds to the arrangement position of the first head chip in thesecond direction; a fourth head chip corresponding to the second ink,formed with nozzles arrayed in the second direction, and arranged at aposition which corresponds to an arrangement position of the second headchip in the second direction and is adjacent to an arrangement positionof the third head chip in the first direction; a first roller arrangedto face the first head chip in the first direction on one side of thesecond head chip in the second direction; and a second roller arrangedto face the third head chip in the first direction on one side of thefourth head chip in the second direction, wherein, when it is assumedthat a nozzle, of nozzles arranged in the second head chip and used forprinting, which is positioned closest to the one side in the seconddirection is referred to as a nozzle A and that a nozzle, of nozzlesarranged in the fourth head chip and used for printing, which ispositioned closest to the one side in the second direction is referredto as a nozzle B, the nozzle A is arranged closer to the one side in thesecond direction than the nozzle B and the first roller is arrangedcloser to the other side in the second direction than the second roller.12. A printer comprising: a first head chip corresponding to a first inkand formed with nozzles which are arrayed in a second directionorthogonal to a first direction in which a recording medium is conveyed;a second head chip corresponding to the first ink, formed with nozzlesarrayed in the second direction, and arranged at a position which isdifferent from an arrangement position of the first head chip in thesecond direction and is adjacent to an arrangement position of the firsthead chip in the first direction; a third head chip corresponding to asecond ink, formed with nozzles arrayed in the second direction, andarranged at a position which corresponds to the arrangement position ofthe first head chip in the second direction; a fourth head chipcorresponding to the second ink, formed with nozzles arrayed in thesecond direction, and arranged at a position which corresponds to anarrangement position of the second head chip in the second direction andis adjacent to an arrangement position of the third head chip in thefirst direction; a first pressing part configured to press the recordingmedium and arranged to face the first head chip in the first directionon one side of the second head chip in the second direction; and asecond pressing part configured to press the recording medium andarranged to face the third head chip in the first direction on one sideof the fourth head chip in the second direction, wherein, when it isassumed that a nozzle, of nozzles arranged in the second head chip andused for printing, which is positioned closest to the one side in thesecond direction is referred to as a nozzle A and that a nozzle, ofnozzles arranged in the fourth head chip and used for printing, which ispositioned closest to the one side in the second direction is referredto as a nozzle B, the nozzle A is arranged closer to the one side in thesecond direction than the nozzle B and the first pressing part isarranged closer to the other side in the second direction than thesecond pressing part.